1. Field of the Invention
The present invention relates to a drive IC which is used to actuate piezoelectric elements for discharging droplets of ink from selected nozzle ports of an ink Jet printhead.
2. Description of the Related Art
Ink jet printers are widely used as a recording device. Typically, an ink jet printer incorporates, a printhead which relies on piezoelectric elements for discharging droplets of ink, and the piezoelectric elements are operated by a group of ICs.
PCT Publication No. WO 95/16568 (corresponding to U.S. patent application Ser. No. 08/505,207 filed Aug. 14, 1995) discloses an ink jet printhead operated by a drive IC which is designed to supply a drive voltage to each of the piezoelectric elements for discharging a droplet of ink when the corresponding bit of the printing data is "1". The drive IC is also designed to supply a standby voltage to each of the piezoelectric elements when the corresponding bit of the printing data is "0". When applied, the standby voltage causes a slight deformation of the piezoelectric element only to prepare for a subsequent larger deformation of the piezoelectric element, but does not cause discharging of ink. Thus, only a single kind of drive voltage is utilized for making a constantly sized droplet of ink.
On the other hand, when an ink jet printer is used to print an image with a gradation like a photograph, the gradation need be represented by changing the density of printing dots. Specifically, a thicker portion of the image may be represented by increasing the dot density, whereas a thinner image portion may be represented by decreasing the dot density. However, a decrease of the dot density makes the individual dots visually outstanding, so that an unnatural image may result particularly with respect to color printing.
The individual dots may be rendered non-outstanding by decreasing the size of the dots. However, if the size of the individual dots is reduced, an increased amount of drive data is required for a unit area of the image. For instance, if the diameter of the dots is halved, the dot area becomes 1/4, requiring four times as much data and time.
Thus, the drive IC disclosed in the above-described PCT publication, due to the adoption of a single kind of drive voltage, suffers a problem of requiring a longer time if the size of the individual dots are reduced to improve the gradation of the printed image. For instance, about 14-25 minutes are required for performing full color printing on an A4-size paper with a density of 720 dpi.